Slowing down a child on a swing is perhaps the most ubiquitous form of feedback cooling, but in recent years the technique has also been widely applied in the field of optomechanics. In well-engineered cavity optomechanical systems, laser light can be used to measure the position of a mechanical oscillator and apply an appropriate feedback force. These previous works relied on an optical cavity to enhance measurement precision and achieved cooling all the way to the quantum ground state. Pluchar et al. take the first step towards achieving cavity-free feedback cooling of a mechanical oscillator to its ground state: using a free-space interferometer they cool a Si₃N₄ membrane by five orders of magnitude, taking it from room temperature to 5 millikelvin (phonon occupation of 3000).  The membrane position is measured using a balanced Michelson interferometer and feedback is applied by radiation pressure with a second, auxiliary light field. The authors project that ground state cooling could be achieved with improved mechanical resonators at moderate cryogenic temperatures (~5K).

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